1. Field of the Invention
This invention concerns a method of radiation induced dry etching of a metallized substrate. More particularly, the invention concerns the use of a specially selected gas that reacts with the metal forming a solid reaction product which is capable of being removed when irradiated with a beam of laser radiation.
2. Description of the Prior Art
The trend in electronics today is towards systems of ever increasing component density. Increased component density permits designers to achieve greater speed and complexity of system performance while maintaining system size at a minimum. Additionally, increased component density enables manufacturers to lower production costs owing to the economies that can be realized using integrated circuit processing.
The desire for increased component density has given rise to very large scale integrated circuit (VLSI). In such circuits, designers pack large numbers of electrical components onto individual integrated circuit chips. Subsequently, these chips are ganged on a substrate to form larger circuits and functional blocks of a system.
To facilitate the mounting of the high density circuit chips, designers have developed the so-called multilayer ceramic (MLC) substrate. The MLC substrate is well known and has been described in such articles as "A Fabrication Technique for Multilayer Ceramic Modules" by H. D. Kaiser et al, appearing in Solid-State Technology, May 1972, pp 35-40.
An example of a semiconductor module including a multilayer ceramic substrate is given in U.S. Pat. No. 4,245,273 issued to Feinberg et al and assigned to the assignee of this application.
MLC manufacturers have found that substrate performance, particularly, the maximum circuit speed the substrate will sustain, can be increased by reducing the length of the thick film metal wiring built into the substrate to interconnect the chips. Designers have proposed to reduce interconnection wiring by replacing at least some of the MLC thick film circuits with multilayer thin film circuits. Particularly, designers have proposed to use thin film circuits at the MLC chip mounting surface. The thin film circuits are formed at the MLC chip mount surface as multiple layers of thin film metal separated by layers of insulation. The multiple metal layers are interconnected by vertical metallization which extends through holes commonly referred to as vias that are arranged in a predetermined pattern.
Because it is possible to make a line of smaller dimension using thin film technology as compared with thick film technology, it is possible to fit more circuits in a substrate plane. Where higher circuit density per plane is achieved, fewer planes are required and accordingly the circuit wiring length interconnecting the multiple planes can be reduced. By shortening the plane interconnection metallization less circuit inductance and parasitic capacitance is present permitting a higher performance frequency. This technique for increasing frequency capability has come to be referred to as Thin Film Redistribution (TFR). An illustration of an MLC including a TFR structure is provided in U.S. Pat. No. 4,221,047 issued to Narken et al and assigned to the assignee of this application.
While the size of TFR multilevel metallization structure is smaller than that of thick film, it is not as small as thin film metallization structure used on the chips. Because the TFR current is a combination of the currents supplied by the multiple chips, it is substantially greater than the chip current. The TFR metallization must therefore be of larger physical size than that of the chip to maintain current densities and associated heating at acceptable levels. Additionally, the dielectric separating the TFR metal layers is also thicker and of different composition. As taught in the above mentioned U.S. patents, copper is the metal most widely used for forming the metallization patterns. It is therefore obvious that copper etching is an essential process in both Thin Film Redistribution (TFR) and Metallized Ceramic Polyimide (MCP) technology and more generally for various packaging applications, where there is a need to define wiring patterns in thick copper films.
Unfortunately, because TFR metallization structures are larger than those of an integrated circuit (I.C.) chip and because the materials are somewhat different, the thin film process techniques conventionally used for I.C. chip metallization fabrication such as the lift off etching technique, and dry etching (plasma or reactive ion etching) cannot be easily used in making TFR structures. The lift off technique is complex and different to define thick films. Dry etching needs complex equipment and process steps involving inorganic masks such as MgO and SiO.sub.2. Furthermore, dry etching is not accurately repeatable and controllable particularly in large batch processing.
On the other hand, new techniques have been recently explored, based primarily on the radiation-enhanced chemical interactions at gas-solid interfaces by energetic particles such as ions, electrons, and photons. Laser radiation, because of its monochromatic and coherent characteristics, can be readily focused or collimated onto solid surfaces and is well-suited to promoting surface reactions.
Techniques of etching various materials at high etch rates by using laser induced dry chemical etching have been reported in the literature. For example, Chuang in his article, "A Laser-Enhanced Chemical Etching of Solid Surfaces", IBM Journal of Research and Development, Vol. 26, No. 2, March 1982 reports that silicon and tantalum can be successfully etched by vibrational excitation of SF.sub.6 using a CO.sub.2 laser. In addition, Chuang notes the etching of silicon by dissociation of SF.sub.6 using a CO.sub.2 laser and the etching of silicon dioxide by dissociation of chlorine gas using an argon laser.
Reference is also made to the article entitled "Surface Etching by Laser-Generated Free Radicals" by Steinfeld et al, published in the Journal of the Electrochemical Society, Vol. 127, No. 1, January 1980, for a description of the etching of SiO.sub.2 by the disassociation of CF.sub.3 Br using a CO.sub.2 laser and the etching of silicon nitride by the disassociation of CF.sub.2 Cl.sub.2 and CDF.sub.3 using a CO.sub.2 laser.
Reference is also made to U.S. Pat. No. 4,260,649 which discloses a method and apparatus for chemical treatment of a silicon wafer wherein said wafer is exposed to a controlled gaseous atmosphere containing a gaseous constituent to be dissociated by laser radiation to produce a gaseous reactant product for reaction with a surface of the wafer. In this method the wavelength of the laser beam radiation is selected exclusively for splitting the desired bonds to produce only the desired reactant product without producing undesired by-products which could deleteriously interfere with the desired chemical reaction. As a specific example, this patent teaches selective etching of a metal over silicon or silicon dioxide by using gaseous BCl.sub.3 at a few Torr pressure and irradiating the gas with a CO.sub.2 laser of wavelength 10.6 .mu.m to dissociate the BCL.sub.3 into BCl.sub.2 and Cl. The Cl component (and perhaps BCl.sub.2) reacts with the metal to selectively etch the metal. Thus, in this method the surface to be treated is not directly irradiated with the laser beam.